The present invention relates to robot controls and more particularly to pulse width modulators employable in digital robot controls to generate control signals for operating the power amplifiers and robot joint motors and thereby controlling the robot arm motion and positioning.
In the referenced patent applications, there is disclosed a digital robot control that provides improved robot operation and performance. In the digital robot control system, a pulse width modulator is employed to interface the digital control signals to conventional commercially available power packs or power amplifiers that drive the joint motors. The joint motor drive current, and thus the motor torque, is determined by amplifier cycle conduction time which is regulated by control pulse width. A single feedback resistor is used with the power pack to provide actual motor current signals for closed loop torque control.
As set forth in referenced W.E. 53,225, the pulse width modulator generally employs a cyclic ramp generator to generate output control pulses that produce amplifier conduction time corresponding to axis voltage commands and thus axis torque commands. For a zero voltage command, forced negative and positive end stop control voltages produce a ripple motor current that enables the regenerative motor current amplitude and polarity to be determined and thereby enables motor current feedback control with the use of economy hybrid power packs having a single power amplifier resistor in brushless and brush type DC motor embodiments.
At zero motor voltage command, any regenerative motor current is forced by switch action to flow through the current sense resistor on the power supply circuit. Although the sense resistor current is always in the same direction, the motor current direction (polarity) at any point in time is known because of the directing effect of the end control pulses on regenerative motor currents. With the use of the end stop pulses, power switches are always turned on at the ends of each PWM cycle to force any regenerative motor current to flow through the power supply and the current sense resistor thereby enables motor current amplitude detection even if the motor voltage command is then zero.
While each end stop pulse exists for a short period of time, it produces a small motor current that persists until the next end stop pulse or control pulse. Similarly, the next end stop pulse produces a small but reverse motor current that persists until the next cycle starts. Thus a small motor ripple current results from successive end stop pulses in the absence of control pulses, but the average value of the ripple current is zero so that it has essentially no effect on the current, position and velocity control of the motor.
The motor ripple current has been found to be a source of audible noise that can be very annoying to operating personnel especially at lower operating frequencies of the pulse width modulator, i.e. where higher load capacity robots are being run by the robot control. In the higher load robot applications, the ripple current also produces greater motor heating. The present invention is directed to reducing or eliminating such audible noise and such added motor heating while retaining the benefits and advantages of the pulse width modulation scheme in the referenced digital robot control prior art.